1. Field of the Invention
The present invention relates to slow positron beam generators and more particularly to beam generators having positron control capable of stopping a positron in materials, including thin films, to be tested.
2. Description of Related Information
Polymers are strong, light-weight, and can be developed to have desirable mechanical, electrical and optical properties. In the aerospace industry, applications for polymers are increasing. Some of the more challenging applications often call for polymers in the form of thin films. For example, polymer thin films make very desirable dielectrics for microelectronics and use in capacitors. Properties of thin films strongly depend on their molecular morphology. Thin films with the same chemical composition and density can have different physical properties depending on their processing history. For many applications, the internal structure of a thin film should be tested before the thin film is used. Therefore, a technique is necessary for providing information about the internal structure of "finished" thin films.
The inventors in the present invention set out to develop a viable technique for testing the internal structure of materials. Especially difficult was the testing of very thin film materials. No positron beam generator was known to accurately test a material at atmospheric pressure. The adaption of conventional positron annihilation spectroscopy (PAS) was tried for studying thin films.
In the prior art, positron annihilation spectroscopy was used for measuring free volume in polymer discs. See, e.g., "Moisture Dependents of Positron Annihilation Spectra in Nylon N-6", Nucl. Inst. and Methods, Vol. 221 427, 1984 by J. J. Singh, T. L. St. Clair, W. H. Holt, and W. Mock, Jr. Also see "Applications of Positron Annihilation Spectroscopy in Materials Research", Proceedings of 33rd International Symposium on Materials--Pathway to the Future, held at Anaheim, Calif., Mar. 7-10, 1988 by J. J. Singh, Edited by G. Carill, E. D. Newell, W. D. Brown and P. Phelam, pp. 470-421, published by SAMPE, Covina, 1988.
FIG. 1 illustrates a multi-degrader source-target assembly built by the inventors as an early attempt to study thin films. Test films 12 are sandwiched between suitable aluminum energy degraders 10 such that positrons of progressively higher energy are forced to stop in the test films. The multi-degrader source-target assembly includes a sodium isotope (Na.sup.22) source 14. Preferably all of the positrons emitted from sodium isotope source 14 will stop in a single test film 12. However, as illustrated by the arrows pointing from the sodium isotope source 14 to the test films 12, emitted positrons stop in multiple films. Positrons also stop in aluminum energy degraders 10. In order to eliminate effects of positrons annihilating in the aluminum energy degraders 10, lifetime spectrums were taken both with and without the test films 12 in the multi-degrader source-target assembly. The difference between the spectrums with the test film in the target assembly and the spectrums without the test film in the target assembly represented the test film lifetime data. However, only a small fraction of the incident positron beam stopped in the test films and produced annihilation radiation 16 due to the original Fermi energy distribution of the sodium isotope (NA.sup.22) positrons. Unfortunately, the assembly of FIG. 1 required 24 hours or more to accumulate enough positron annihilation event data for adequate statistics, even with a 25 .mu.c (microurie) Na.sup.22 source.